The field of the invention is autonomous navigation systems for determining position based on inputs of magnetic field and attitude data without the use of Kalman Filters. The present invention may be used for earthbound objects such as submarines and balloons as well as for spacecraft.
In the context of autonomous spacecraft navigation, embodiments of the present invention are particularly useful in the context of low earth orbiting (LEO) spacecraft. All spacecraft navigation systems, at times, cannot determine their location. When this occurs, a spacecraft may have to rely on ground operations personnel to upload a position fix via a communications link. Because of the lack of navigation data, this communications link may be difficult for the spacecraft to establish. If the spacecraft cannot recover its orbit, instruments onboard the spacecraft may become damaged, or worse, the spacecraft maybe lost. To avoid this situation, systems for a spacecraft to autonomously recover its position data, meaning without being supplied information externally, have been developed.
One solution would be to put Global Positioning System Receivers on spacecraft. However, space qualified GPS Receivers have not as of the date of filing of this application been successfully demonstrated in space. In addition, the power increase of fifteen Watts and the expense would be impractical for many spacecraft, particularly small LEO spacecraft. Moreover, many LEO spacecraft, such as those of the Small Explorer (SMEX) Program at the NASA Goddard Space Flight Center, only need to know their positions to within one hundred (100) kilometers, for contacting ground stations for instance, rather than to the accuracy in the hundreds of meters provided by GPS Receivers.
Instead of putting new hardware on small spacecraft, software solutions used in conjunction with already existing hardware on spacecraft have been explored. One such method of determining position onboard a spacecraft used a transponder with either tracking satellite or ground station data. (See Gramling, C., et al, "TDRSS Onboard Navigation System (TONS) Flight Qualification Experiment," Proceedings of the 1994 Flight Mechanics and Estimation Theory Symposium, Greenbelt, Md., May 1994, pp. 253-267 and Gramling, C. "Autonomous Navigation Integrated with NASA Communication Systems," Proceedings of the 19th Annual AAS Guidance and Control Conference, Breckenridge, Colo., February, 1996, Paper No. AAS 96-006.) Although the method is cost effective for transponder users, it can take 12 hours to a full day to converge on an initial position solution due to poor visibility. The method is also Kalman Filter based so that it requires a priori position information.
Software methods to autonomously determine position, have been developed to use magnetic field data provided by magnetometers, which are already required by most spacecraft for momentum management. Methods have been developed which sequentially filter magnetic field data provided by magnetometers with extended Kalman Filters. Position estimates with the use of Kalman Filters have been shown to provide positions to better than fifty 50 km using flight data, (See Shorsi, G., and Bar-Itzhack, I., "Satellite Autonomous Navigation Based on Magnetic Field Measurements", Journal of Guidance, Control, and Dynamics, Vol. 18, No, 4, July-August 1995, pp. 843-850) and even to better than one 1 km using simulated star tracker attitude information with simulated magnetic field measurements. (See Psiaki, M., "Autonomous Orbit and Magnetic Field Determination Using Magnetometer and Star Sensor Data," August, 1993, AIAA-93-3825-CP.)
However, these Kalman Filtering methods require an a priori knowledge of a position fix of the spacecraft for the filters to converge. Furthermore, the filters may take more than one hundred (100) minutes to converge on a solution and require extensive computing power. Computing power on a spacecraft must be shared with other necessary systems. For example, computing power onboard a SMEX spacecraft must be shared with every other subsystem, so it is safe to assume that only five 5 percent is available for the position determination process. In Psiaki, four iterations for a good guess and 25 iterations for a poor first guess (.about.2700 km error) which took 100 minutes, were obtained on a one 1 Mflop workstation. An upper end SMEX only has about 900 Kflops.
Obtaining a position fix of within 100 km accuracy in significantly less time than 100 minutes based on magnetometer data and attitude data without a priori knowledge of position is highly desirable in several contexts. For LEO spacecraft, the spacecraft would quickly be able to determine its position and with that knowledge, recover its orbit. A quickly obtained position fix would provide a good first guess as the a priori knowledge of position needed by a Kalman Filter based method, thereby decreasing time to convergence. Furthermore, a fine positioning spacecraft using GPS could also benefit from a real time coarse position estimate to decrease its time to a first position fix or offer a first when less than four (4) GPS space vehicles are visible.